Microelectronic chips are minute sized electronic circuits as may be formed of a number of different materials, including organic material, superconducting material and semiconductor material, to provide electronic functions, such as amplification, oscillation and the like. Although the materials and fabrication for those devices may differ, the result is a small or minute size device, referred to as a chip.
By way of example, those microelectronic chips, referred to as semiconductor chips or, more accurately, semiconductor dies, are formed within a large wafer of material that, following processing, is cut apart into a large number of individual semiconductor dies of rectangular shape. Each of those dies contains the particular electrical properties desired of a particular electronic component or device, and the die may form any of the familiar electronic devices specified by the semiconductor device designer, such as, by way of example, microprocessors, random access memory, digital processors, R.F. amplifiers, and the like, the specific device not being relevant to the present invention. Other examples are found in the small devices manufactured of superconducting material.
Although reference, such as the foregoing reference, may be made herein to semiconductor chips, the reader is cautioned that specific reference herein to same should not be construed to limit the scope of the present invention to chips formed of semiconductor material. As the detailed description that follows makes clear the present invention has application to all such electronic chips, irrespective of the physical principles underlying the functional electronic properties of the particular chip.
As fabricated, each microelectronic chip also contains a number of metal spots, suitably of gold or aluminum, termed connection sites or bonding pads, usually arranged or spaced apart in one or two rows along a relatively flat upper surface. By means of those bonding pads, the microelectronic chip may be electrically connected in electrical circuits with other electronic devices, either for receiving and/or sending electrical signals, and with a D.C. source that provides electrical power to operate the device.
Before the microelectronic chip is industrially useable, however, it is packaged in a protective container or, as variously termed, package. That package also contains the electrical contacts, terminals, or prongs, as variously termed, that extend through the package for connection to external electrical circuits customarily formed on a printed circuit board. Inside the package, the connection sites or bonding pads on the chip, are electrically connected to a corresponding one of those electrical terminals or prongs, allowing the chip, so to speak, to communicate with the outside world. Those internal electrical connections may be formed in a number of different known ways: cold welding, wire bonding or solder bonding.
In most cases, an individual microelectronic chip is assembled into its own container or package, such as the known plastic package or the Cermet package. An important clarification is given to avoid any confusion for the reader. As individually packaged, the microelectronic device is also customarily referred to generally as a microelectronic chip. To avoid confusion due to that dual use of the terminology, the reader should understand that when reference is made to microelectronic chip in the description of the present invention, unless otherwise noted, the reference is intended to an unpackaged chip.
Continuing with this background, a single packaged chip is typically inserted into a printed circuit board, along with other individual packaged chips and other electrical components, and is soldered in place. If for any reason, a chip on that circuit board fails, the circuit board is easily fixed or, as variously termed, reworked. The chip is unsoldered and removed, and another identical chip is soldered back in place on the circuit board. Since individually packaged chips are inexpensive and are not designed to be repaired, the failed chip is conveniently discarded.
Alternatively, for reasons not here relevant and not discussed, a quantity of two or more microelectronic chips and associated circuits are often assembled together in a single package, referred to as a multi-chip module (MCM). Typically an MCM package contains two, three, or more microelectronic chips and associated circuits, which are supported in common on a base. The base, typically a circuit board formed of ceramic or laminated substrate, supports and includes the electrical circuit paths for the microelectronic chips and ancillary components, and contains the signal and power leads that extend to the outer edges of the base; and also serves as a bottom wall to the module package. A continuous side wall, referred to as a seal ring, seals the base's upper surface and surrounds the region occupied by the microelectronic chips and ancillary components. The signal and power conductors, are routed through intermediate layers of the base's multiple layers and under that seal ring to the outside of the side wall and on all four sides of the seal ring. Electrical terminals, located outside the seal ring, connect to those signal and power conductors and extend outwardly, enabling the module to be connected to external circuitry. The module is closed by a metal cover or lid that typically is hermetically sealed to the seal ring. It is appreciated that the multi-chip module is a complex electronic device and its performance benefits are achieved at significant manufacturing cost.
Cold welding, more specifically, Indium cold welding is presently becoming the bonding technique of choice for bonding the microelectronic chips into a multi-chip module when exposure of sensitive circuit elements to elevated temperatures cannot be tolerated. It is a simple, low cost approach to accomplish bonding. In this technique, metal pads of equal size and composition, preferably Indium, are deposited at the connection sites on both the microelectronic chip and the multi-chip module's printed circuit board. The deposit is of a suitable thickness, as example, between twelve and three-hundred microns. Pure Indium is preferred as the deposited metal since it is very soft and deforms easily. The chip is turned over, that is, "flipped" over, a basis for the "flip chip" terminology, so that the connection sites on the chip face the corresponding connection sites or bonding pads on the substrate. The connection sites on the chip are aligned with associated connection sites on the other element, and the two elements are pressed together at about room temperature.
As a consequence Indium from both the flip chip's connection sites and the sites on the substrate diffuse into each other, creating a mechanical and electrical bonds, referred to as cold weld joints. The foregoing produces a cold welded microelectronic chip. Thus, when one skilled in the art makes reference to a cold welded microelectronic chip, and as that term is used in the following description, the reference is to the foregoing structure. The cold weld process is simple and low cost. However, it has one basic flaw in this application. In case of a microelectronic chip failure, the multi-chip module module cannot be reworked.
As one appreciates, the multi-chip module contains many chips and other electrical components. It is a much much more expensive component to produce than a single packaged microelectronic chip. Until the present invention, if for any reason one of the chips in the multi-chip module fails, the entire module must be removed and replaced with another, a very expensive proposition, unless it is possible to rework the multi-chip module, which, as hereafter explained, has not been the case with cold welded multi-chip modules. The inability to rework cold welded multi-chip modules, required construction of additional reserve replacement modules, spares, when a single multi-chip module is purchased, and adds to the great expense.
The inability to rework is due to the metal diffusion that formed the cold weld joint. When the cold weld joint is broken to remove the microelectronic die or as variously termed, chip, from the printed circuit board to fix or rework the module, the place in the cold weld material at which the weld tears is uncontrollable, that is, random in nature. For example, the Indium joint could break at the interface with the chip pad leaving no Indium on the microelectronic chip connection site, or break at the interface with the substrate pad, leaving no Indium on the substrate, or at some location in-between those two extremes. Because the amount of Indium remaining on the chip connection sites and the substrate pad sites is unpredictable and cannot be controlled, rework is impossible.
Consider for example a microelectronic die containing two rows of connection sites with twenty connection sites in each row that is Indium cold welded to corresponding sites on the substrate, the printed circuit board. When the suspect chip is torn away from the substrate, the connection sites on the substrate will contain residual Indium that extends vertically to different levels of height, and one or more sites may have no Indium at all. A fresh replacement chip contains connection sites having Indium deposited uniformly to a predetermined height or depth as variously termed. Thus were one to seek to cold weld that fresh replacement chip to the substrate, some of those sites on the chip would not sufficiently contact some of the shorter height connection sites on the substrate, and, in those situations in which the Indium was completely stripped off a connection site on the substrate, there would be no mechanical or electrical contact whatsoever. Should only one of the sites on the chip fail to make a physical and electrical connection to a corresponding site on the substrate, the electrical circuit fails and the multi-chip module is a complete loss.
Further, the connection sites on the suspect chip which was torn away also contain different heights of residual Indium and perhaps at least one site in which the Indium is torn away completely, the exact configuration being unpredictable due to the random nature of the tearing. Should one desire to return that same chip and try to cold weld it back in place, one would find that at the site at which the Indium was completely torn away, a successful cold welded joint cannot be reconstructed. And again the module fails and would have to be discarded. Thus it is not possible to remove a cold welded flip chip from the circuit board substrate to perform electrical tests on that flip chip and thereafter replace that flip chip on the circuit board.
Although the cold weld process is a simple, low cost approach to flip chip to substrate bonding, the inability to perform rework makes the cost of multi-chip module's prohibitive in many cases.
To provide a reworkable cold weld multi-chip module, one might initially make reference to one of the other microelectronic bonding techniques, only to find other seemingly insurmountable obstacles of a different nature. Wire bonding was initially used to electrically interconnect microelectronic dies to a substrate. Wires were extended between the connection sites on the upper surface of the microelectronic chip and the metal pads on the substrate that supports the chip. For rework, one need only cut the connecting wires and remove the failed chip, and rewire a replacement chip in place. That bonding technique, however, does not prove satisfactory in high speed digital or high frequency applications. Due to their great length, the wires exhibit too much electrical inductance, and that high inductance limits the speed at which the electrical signals may travel.
Solder bonding offered another alternative. In this bonding technique the chip was turned upside down, the origin for naming the construction as "flip-chip", and the chip's electrical connection sites, are soldered directly to aligned metal bonding pads deposited on the circuit board. This technique avoids lengthy wires as in the case of wire bonding. To accomplish soldering, minute solder balls are deposited on the metal bonding pad and the entire assembly, including the microelectronic chips, are heated to solder the flip chip's contacts to the bonding pad, producing a mechanical and electrical connection. That bonding technique proved satisfactory and it is used to fasten flip chips in place even in the multi-chip modules.
When a flip chip in the multi-chip module of this construction is found to be inoperative or fails in service, the multi-chip module could be reworked. That is, by reheating the multi-chip module assembly sufficiently to cause the solder to reflow, the suspect chip could be could be pulled away and removed. A replacement chip could then be substituted in its place and the solder again reflowed to mechanically and electrically join the new flip chip to the base.
However, a typical solder used in that application reflows at about 200 degrees Centigrade, a temperature which most microelectronic devices, including semiconductors, could easily tolerate. Unfortunately, not all semiconductors and other microelectronics used in high performance multi-chip modules are capable of withstanding those solder reflow temperatures. The solder bonding approach is not a viable alternative for temperature sensitive multi-chip modules. The solder bonding technique is thus limited to those circuits which are able to withstand elevated temperatures.
Thus, prior to the present invention, no technique existed for reworking the cold welded multi chip modules, either to replace a defective chip or remove a chip for test and later return it to the module, if the flip chip was connected by an Indium cold weld joint, that is, was Indium cold welded to the module's base.
The lack of a viable alternative bonding technique and the inability to perform rework of cold welded microelectronics makes the cost of high performance multi-chip module's prohibitive in many cases, the dilemma that faced the applicants and led to the present invention. The present invention solves the problem of rework of high performance cold welded multi-chip modules. The present invention no longer requires one to discard a cold welded multi-chip module merely because a chip in that module has failed, thereby providing a significant economic advantage in this field.
The present invention offers a novel approach to performing rework on cold weld flip chip assemblies. The invention permits an individual non-functioning flip chip in a multi chip module to be removed and replaced with a working flip chip. By making rework possible, multi-chip modules containing one failed chip may now be salvaged.
Accordingly, the principal object of the invention is to make possible the rework of cold welded multi-chip modules and thereby salvage such modules.
Another object of the invention is to permit construction of high performance cold welded multi-chip modules at lower cost and with greater efficiency than heretofore.
A further object is to define a new technique for rework of cold welded multi-chip modules.
An ancillary object of the invention is to provide a cold weld joint that severs or breaks in a predictable manner when the joint is manually broken.